Method of precision beam collimation using fiber-optic circulator and wavelength tunable source

ABSTRACT

A method of calibrating a collimating lens system includes transmitting, using an optical transmitter, a beam out of an optical fiber and through a collimating lens of the collimating lens system. The beam is reflected off a perfect flat mirror positioned at an output of the collimating lens and back towards the collimating lens, and received, via the collimating lens, at a power meter connected to the optical fiber. The method also includes adjusting a position of a tip of the optical fiber proximal to the collimating lens while tracking a power reading using the power meter, selecting a calibration position of the optical fiber corresponding to a highest power reading, and securing the optical fiber relative to the collimating lens using the calibration position.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/994,927, filed Aug. 17, 2020, the entire disclosure of whichis incorporated by reference herein.

BACKGROUND

Information can be transmitted over directional point-to-point networks,such as aerospace and other mobile networks. In such networks, links canbe formed between pairs of nodes, or terminals at each node, by aiminglens systems of each node pair towards each other. In someimplementations, the nodes may transmit and receive optical signalsthrough free space optical communication (FSOC) links.

BRIEF SUMMARY

Aspects of the disclosure provide for a method of calibrating acollimating lens system. The method includes transmitting, using anoptical transmitter, a beam out of an optical fiber and through acollimating lens of the collimating lens system; reflecting the beam offa perfect flat mirror positioned at an output of the collimating lensand back towards the collimating lens; receiving, via the collimatinglens, the reflected beam at a power meter connected to the opticalfiber; adjusting a position of a tip of the optical fiber proximal tothe collimating lens while tracking a power reading using the powermeter; selecting a calibration position of the optical fibercorresponding to a highest power reading; and securing the optical fiberrelative to the collimating lens using the calibration position.

In one example, the collimating lens system includes a collimator and anafocal telescope. In another example, the reflecting of the beamincludes adjusting the perfect flat mirror using a tip-tilt mount so thebeam is reflected back towards the collimating lens. In a furtherexample, the adjusting of the position includes moving the optical fiberalong a center axis of the collimating lens. In yet another example, theadjusting of the position of the tip of the optical fiber includes usingone or more processors to control a motor that is connected to the tipof the optical fiber.

In a still further example, the securing of the optical fiber includesfixing the optical fiber relative to the collimating lens in thecalibration position. In another example, the method also includespositioning an afocal telescope at the output of the collimating lens;adjusting a position of one or more lenses of the afocal telescope whiletracking a second power reading using the power meter; selecting asecond calibration position of the one or more lenses corresponding to ahighest second power reading; and securing the one or more lenses usingthe second calibration position. In a further example, the method alsoincludes assembling the collimating lens system into an opticalcommunication device after securing the optical fiber relative to thecollimating lens.

Other aspects of the disclosure provide for a calibration system for acollimating lens system. The calibration system includes a perfect flatmirror positioned to reflect a first beam transmitted by the collimatinglens system back to the collimating lens system, an optical transmitter,a power meter, and an optical circulator having a first port configuredto receive a second beam from the optical transmitter, a second portconfigured to output the second beam and receive the first beam from thecollimating lens system, and a third port configured to output the firstbeam to the power meter.

In one example, the perfect flat mirror is positioned using a tip-tiltmount. In another example, the optical transmitter is a wavelengthtunable laser. In a further example, the calibration system alsoincludes one or more processors configured to move an optical fiber ofthe collimating lens system relative to a collimating lens of thecollimating lens system; track a power reading for each position of theoptical fiber using the power meter; and select a calibration positionof the optical fiber corresponding to a highest power reading. In thisexample, the calibration system optionally also includes a motorconfigured to move a tip of the optical fiber. Further in this example,the calibration system optionally also includes a mechanical armconnecting the tip of the optical fiber with the motor. Also in thefurther example, the one or more processors are optionally configured tomove the optical fiber until the calibration position is selected.

Further aspects of the disclosure provide for a tangible, non-transitorycomputer-readable storage medium configured to store instructions. Theinstructions, when executed by one or more processors, cause the one ormore processors to perform a method. The method includes transmitting,using an optical transmitter of a calibration system, a beam through alens system towards a perfect flat mirror, wherein the beam is reflectedoff the perfect flat mirror and back through the lens system; adjusting,using a motor, a position of a tip of an optical fiber of the lenssystem proximal to a collimating lens of the lens system; tracking apower reading of the reflected beam using a power meter that receivesthe reflected beam from the optical fiber of the lens system; andselecting a calibration position of the optical fiber corresponding to ahighest power reading.

In one example, the method further comprises holding the optical fiberin the calibration position. In another example, the adjusting of theposition includes moving the tip of the optical fiber along a centeraxis of the collimating lens. In a further example, the method alsoincludes adjusting the perfect flat mirror to reflect the beam backthrough the lens system. In this example, the adjusting of the perfectflat mirror optionally includes moving the perfect flat mirror using atip-tilt mount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial diagram of a lens system in accordance withaspects of the disclosure.

FIG. 2 is a functional diagram of a communication device in accordancewith aspects of the disclosure.

FIG. 3 is a functional diagram of a network in accordance with aspectsof the disclosure.

FIG. 4 is a pictorial diagram of a calibration system in accordance withaspects of the disclosure.

FIG. 5 is a functional diagram of another calibration system inaccordance with aspects of the disclosure.

FIG. 6 is a pictorial diagram of a further calibration system inaccordance with aspects of the disclosure.

FIG. 7 is a flow diagram of an example operation in accordance withaspects of the disclosure.

DETAILED DESCRIPTION Overview

The technology relates to calibrating a lens system so that beamstransmitted by the lens system are collimated. The lens system may befor an optical communication terminal, an afocal telescope, or othertype of device where collimated beams are required for transmission orreceipt. The calibration may be performed using a perfect flat mirrorduring manufacture of the device.

The calibration system and method described herein may produce veryprecise collimators. The proposed calibration is indirectlyquantitative, inferring wavefront precise focus alignment using receivedpower. In addition, the proposed calibration may be performed on anywavelength beam and any size beam as long as a large enough perfectlyflat mirror is used. Sourcing a perfectly flat mirror may be lessexpensive than sourcing interferometers or wavefront sensors. Automationand scaling for production may be easily achieved at a low cost.

Example Systems

A lens system may include a collimating lens or lens group and a singlemode optical fiber. For example, the lens system may be a collimator.The collimating lens or lens group may be configured to receive a beamand direct the beam to a tip of the optical fiber. A beam transmittedfrom the optical fiber may be collimated using the collimating lens orlens group and transmitted out into free space. Shown in FIG. 1 is alens system 100 that includes a collimating lens 110 and an opticalfiber 120. The optical fiber 120 has a tip 122 proximal to thecollimating lens, positioned to receive a beam via the collimating lens110 and direct a beam towards the collimating lens 110.

The lens system may be assembled in a device, such as an opticalcommunication device shown in FIG. 2. The optical communication device200 may be configured to form one or more communication links with otheroptical communication devices. The optical communication device mayinclude one or more processors 220, a memory 230, and one or moretransceivers 240. The one or more transceivers may include the lenssystem 100 and a photodetector 246.

The one or more processors 220 may be any conventional processors, suchas commercially available CPUs. Alternatively, the one or moreprocessors may be a dedicated device such as an application specificintegrated circuit (ASIC) or other hardware-based processor, such as afield programmable gate array (FPGA). Although FIG. 2 functionallyillustrates the one or more processors 220 and memory 230 as beingwithin the same block, it will be understood that the one or moreprocessors 220 and memory 230 may actually comprise multiple processorsand memories that may or may not be stored within the same physicalhousing. Accordingly, references to a processor or computer will beunderstood to include references to a collection of processors orcomputers or memories that may or may not operate in parallel.

Memory 230 stores information accessible by the one or more processors220, including data 232 and instructions 234 that may be executed by theone or more processors 220. The memory may be of any type capable ofstoring information accessible by the processor, including acomputer-readable medium such as a hard-drive, memory card, ROM, RAM,DVD or other optical disks, as well as other write-capable and read-onlymemories. The system and method may include different combinations ofthe foregoing, whereby different portions of the instructions and dataare stored on different types of media.

Data 232 may be retrieved, stored or modified by the one or moreprocessors 220 in accordance with the instructions 234. For instance,although the system and method are not limited by any particular datastructure, the data 232 may be stored in computer registers, in arelational database as a table having a plurality of different fieldsand records, XML documents or flat files.

Instructions 234 may be any set of instructions to be executed directly(such as machine code) or indirectly (such as scripts) by the one ormore processors 220. For example, the instructions 234 may be stored ascomputer code on the computer-readable medium. In that regard, the terms“instructions” and “programs” may be used interchangeably herein. Theinstructions 234 may be stored in object code format for directprocessing by the one or more processors 220, or in any other computerlanguage including scripts or collections of independent source codemodules that are interpreted on demand or compiled in advance.Functions, methods and routines of the instructions 234 are explained inmore detail below.

The one or more transceivers 240 may be configured to transmit a beamvia the optical fiber 120 and the collimating lens 110 out into freespace. In addition, in the one or more transceivers 240, the opticalfiber 120 may be configured to receive light, such as a beam transmittedfrom a remote communication device, via the collimating lens 110. Theoptical fiber 120 may also be configured to relay the received beamtowards the photodetector 246. The photodetector 246 may be configuredto detect light received at the surface of the photodetector, such asfrom the beam, and may convert the received light into an electricalsignal using the photoelectric effect.

The one or more transceivers 240 may be configured to transmit andreceive optical frequencies via cable, fiber, or free space. One or moreadditional transceivers may also be included that are configured totransmit and receive radio frequencies or other frequencies. The one ormore transceivers 240 are configured to communicate with one or moreother communication devices via one or more communication links. In FIG.3, the communication device 200 is shown having communication links(illustrated as arrows) with client device 310 and communication devices320, 322, and 324.

With a plurality of communication devices, the communication device 200may form a communication network, such as network 300 in FIG. 3. Thenetwork 300 includes client devices 310 and 312, server device 314, andcommunication devices 200, 320, 322, 324, and 326. Each of the clientdevices 310, 312, server device 314, and communication devices 320, 322,324, and 326 may include one or more processors, a memory, and one ormore transceivers. The one or more processors may be any well-knownprocessor or a dedicated controller similar to the one or moreprocessors described above. The memory may store information accessibleby the one or more processors, including data and instructions that maybe executed by the one or more processors. The memory, data, andinstructions may be configured similarly to memory 230, data 232, andinstructions 234 described above. Using the one or more transceivers,each communication device in network 300 may form at least onecommunication link with another communication device, as shown by thearrows. The communication links may be for optical frequencies, radiofrequencies, other frequencies, or a combination of frequency bands.

The lens system 100 may be calibrated using a calibration system, suchas the calibration system 400 shown in FIGS. 4 and 5. The calibrationsystem 400 may include a perfect flat mirror 410 positioned at an outputend of the lens system. The perfect flat mirror 410 may be positioned atleast approximately orthogonal to a beam transmitted from the lenssystem such that a beam transmitted from the lens system may bereflected back to the lens system. In some implementations, the perfectflat mirror may be on a tip-tilt mount so the angle of the mirror may beeasily adjusted relative to a position of the lens system. Thecalibration system may also include an optical transmitter 420, such asa laser, configured to transmit a beam through the optical fiber and thelens system, and a power meter 430 configured to receive a beam from theoptical fiber 120 of the lens system. Alternatively, a near-perfect flatmirror with known flatness variations may be utilized in place of theperfect flat mirror.

In some implementations, the optical fiber 120, the optical transmitter420, and the power meter 430 may be connected using an opticalcirculator 440. The optical fiber 120 may be connected to a first portof the optical circulator 440; the power meter 430 may be connected to asecond port of the optical circulator 440 configured to output a beamreceived at the first port; and the optical transmitter 420 may beconnected to a third port of the optical circulator 440 configured tooutput a beam at the first port.

In another example, a calibration system 500 may also include amechanical arm 510, such as a gimbal, configured to hold the opticalfiber 120 and move the tip 122 of the optical fiber in relation to thecollimating lens 110. The calibration system 500 may additionallyinclude one or more processors 520, a memory 530 including data 532 andinstructions 534, and a motor 540. The motor 540 may be connected to themechanical arm 510 and configured to move the mechanical arm 510. Theinstructions 534 may include instructions executable by the one or moreprocessors 520 to move the optical fiber 120 by controlling the motor540.

The one or more processors may be any well-known processor or adedicated controller similar to the one or more processors describedabove. The memory may store information accessible by the one or moreprocessors, including data and instructions that may be executed by theone or more processors. The memory, data, and instructions may beconfigured similarly to memory 230, data 232, and instructions 234described above.

Example Operations

In addition to the operations described above and illustrated in thefigures, various implementations and methods will now be described. Itshould be understood that the described operations and steps do not haveto be performed in the precise order provided below. Rather, variousoperations and steps can be handled in a different order orsimultaneously, and operations and steps may also be added or omitted.

To calibrate a lens system, such as the lens system 100, the opticaltransmitter 420 in the calibration system 400 may transmit a beam out ofthe optical fiber 120. The beam may be transmitted through the lenssystem 100, towards the perfect flat mirror 410. The beam reflects offthe perfect flat mirror 410 back towards the lens system 100. Thereflected beam may be received at the optical fiber 120 and directed tothe power meter 430 of the calibration system.

The lens system 100 may be adjusted to determine a calibratedconfiguration that produces the highest amount of power received at thepower meter 430. The adjustment to the lens system 100 may includeadjusting a position of the optical fiber 120 in any direction relativeto the collimating lens 110. For example, the adjustment may be made bymoving a tip 122 of the optical fiber closer to or farther away from thecollimating lens 110 along a center axis of the collimating lens. Thecenter axis may also be collinear with a focal point of the collimatinglens 110. The adjustment may also be made additionally or alternativelyby moving the tip 122 of the optical fiber closer to or farther awayfrom the center axis. At each position of the tip 122 of the opticalfiber along the center axis, a power reading may be determined using thepower meter 430. A position that corresponds to a highest power readingmay be selected for use in the calibrated configuration of the lenssystem 100.

In some implementations, the adjustment of the optical fiber 120 may beautomated using the calibration system 500 that has the one or moreprocessors 520 and the motor 540. The one or more processors 520 may beconfigured to move the tip 122 of the optical fiber in along the centeraxis of the collimating lens 110 using the motor 540. While moving thetip 122 of the optical fiber, the one or more processors 520 may obtaina power reading for each point along the center axis. The one or moreprocessors 520 may select a position of the tip 122 of the optical fiberthat corresponds to the point having the highest power reading. Once theposition of the tip 122 of the optical fiber corresponding to the pointhaving the highest power reading is selected, the adjustment of theoptical fiber may be stopped.

The lens system 100 may be secured in the calibrated configuration. Forexample, the tip 122 of the optical fiber and the collimating lens 110may be secured according to the selected position of the optical fiber120 relative to the collimating lens 110. The lens system 100 may besecured using mechanical means, such as screws or bolts, and/oradhesives. The collimating lens 110 may be secured by being mounted on aprecision tip-tilt mechanism.

The calibrated lens system 100 may then be assembled in a device. Forexample, the calibrated lens system 100 may be assembled in the opticalcommunication device 200 described above as part of the one or moretransceivers 240.

Alternatively, the calibrated lens system may be used to calibrateanother lens system requiring collimation, such as an afocal telescope.As shown in FIG. 6, the calibrated lens system 100 may be connected tothe optical transmitter 420 and the power meter 430 and positionedpointed towards the mirror 410 similar to the set up in FIG. 4. Anafocal telescope 600 including a first lens 602 and a second lens 604may be positioned between the mirror 410 and the calibrated lens system100. Adjustments to the first lens 602, the second lens 604, or othercomponent of the afocal telescope 600 may be performed while tracking apower reading using the power meter 430, in a same or similar way asdescribed above with respect to adjusting the tip 122 of the opticalfiber of the lens system 100. A calibrated configuration for the afocaltelescope 600 may be the configurations of the first lens 602, thesecond lens 604, and/or other adjusted components that corresponds to ahighest power reading.

In some alternative implementations, a wavelength tunable laser may beutilized as the optical transmitter 420 of the calibration system. Afocus of the lens system 100 may be calibrated using the wavelengthtunable laser with minimum iteration. For example, one or more lenses ofthe lens system 100 may have an amount of residual chromatic aberration,causing dispersion of light in the one or more lenses. The focal lengthof the one or more lenses may change a known amount with varyingwavelengths due to the dispersion. While the collimating lens 110 andthe optical fiber 120 of the lens system are at a set distance, thewavelength tunable laser may output a beam and vary the wavelength ofthe beam. While the wavelength of the beam is varied, a power readingmay be determined using the power meter 430. A wavelength thatcorresponds to a highest power reading may be determined. Based on adifference between the determined wavelength and a desired wavelengthfor the lens system 100, a distance adjustment for the optical fiber 120relative to the collimating lens may be determined based on the knownamount of change of the focal length between the wavelengths.

In FIG. 7, flow diagram 700 is shown in accordance with some of theaspects of the calibration process described above. While FIG. 7 showsblocks in a particular order, the order may be varied and that multipleoperations may be performed simultaneously. Also, operations may beadded or omitted.

At block 710, a beam may be transmitted through a lens system using anoptical transmitter. At block 720, the beam may be received at a powermeter via the lens system after it is reflected off a perfect flatmirror. At block 730, a position of an optical fiber of the lens systemmay be adjusted relative to a lens of the lens system while tracking apower reading using the power meter. At block 740, a calibrationposition for the optical fiber corresponding to a highest power readingmay be selected. At block 750, the optical fiber may be secured relativeto the lens in the calibration position.

The features described herein may produce very precise collimators. Theproposed calibration is indirectly quantitative, inferring precise focusalignment using received power. In addition, the proposed calibrationmay be performed on any wavelength beam and any size beam as long as alarge enough perfectly flat mirror is used. Sourcing a perfectly flatmirror may be less expensive than sourcing interferometers or wavefrontsensors. Automation and scaling for production may be easily achieved ata low cost.

Unless otherwise stated, the foregoing alternative examples are notmutually exclusive, but may be implemented in various combinations toachieve unique advantages. As these and other variations andcombinations of the features discussed above can be utilized withoutdeparting from the subject matter defined by the claims, the foregoingdescription of the embodiments should be taken by way of illustrationrather than by way of limitation of the subject matter defined by theclaims. In addition, the provision of the examples described herein, aswell as clauses phrased as “such as,” “including” and the like, shouldnot be interpreted as limiting the subject matter of the claims to thespecific examples; rather, the examples are intended to illustrate onlyone of many possible embodiments. Further, the same reference numbers indifferent drawings can identify the same or similar elements.

1. A method of calibrating a collimating lens system, the methodcomprising: transmitting, using an optical transmitter, a beam out of anoptical fiber and through a collimating lens of the collimating lenssystem; receiving, via the collimating lens, the beam back at a powermeter connected to the optical fiber; adjusting, using a motor connectedto the optical fiber, a position of a tip of the optical fiber proximalto the collimating lens while tracking a power reading using the powermeter; and ending the adjusting of the position of the tip of theoptical fiber based on the power reading.
 2. The method of claim 1,wherein the adjusting of the position includes using a mechanical armconnecting the motor and the optical fiber.
 3. The method of claim 1,wherein the adjusting of the position of the tip of the optical fiberincludes moving the optical fiber along a center axis of the collimatinglens.
 4. The method of claim 3, wherein the adjusting of the position ofthe tip of the optical fiber includes detecting a plurality of powerreadings using the power meter at different positions along the centeraxis.
 5. The method of claim 4, wherein the ending of the adjustingincludes positioning the tip of the optical fiber at a calibrationposition where a highest power reading of the plurality of powerreadings is detected.
 6. The method of claim 5, further comprisingassembling the collimating lens system into an optical communicationdevice, the assembled collimating lens system including the opticalfiber in the calibration position relative to the collimating lens. 7.The method of claim 1, further comprising: transmitting the beam throughan afocal telescope after transmitting through the collimating lens;receiving the beam through the afocal telescope before receiving thebeam via the collimating lens; adjusting a position of one or morelenses of the afocal telescope while tracking the power reading usingthe power meter; and ending the adjusting of the position of the one ormore lenses based on the power reading.
 8. The method of claim 1,further comprising using one or more processors to perform the adjustingof the position of the tip of the optical fiber using the motor.
 9. Acalibration system for a collimating lens system, the calibration systemcomprising: an optical transmitter; a power meter; and one or moreprocessors configured to: adjust an optical fiber of the collimatinglens system relative to a collimating lens of the collimating lenssystem while the optical transmitter transmits a beam; detect aplurality of power readings for different positions of the optical fiberusing the power meter; and stop adjusting the optical fiber based on theplurality of power readings.
 10. The calibration system of claim 9,further comprising a motor configured to adjust a tip of the opticalfiber.
 11. The calibration system of claim 10, further comprising amechanical arm connecting the tip of the optical fiber with the motor.12. The calibration system of claim 11, wherein the one or moreprocessors are configured to adjust the optical fiber along a centeraxis of the collimating lens.
 13. The calibration system of claim 12,wherein the one or more processors are configured to stop adjusting theoptical fiber when the optical fiber is positioned at a calibrationposition where a highest power reading of the plurality of powerreadings is detected.
 14. The calibration system of claim 9, wherein theone or more processors are configured to: adjust a position of one ormore lenses of an afocal telescope while the optical transmittertransmits the beam; and ending the adjusting of the position of the oneor more lenses based on the plurality of power readings.
 15. A tangible,non-transitory computer-readable storage medium configured to storeinstructions, the instructions, when executed by one or more processors,cause the one or more processors to perform a method, the methodcomprising: transmitting, using an optical transmitter of a calibrationsystem, a beam through a lens system including an optical fiber and acollimating lens; adjusting, using a motor, a position of a tip of theoptical fiber relative to the collimating lens; detecting a plurality ofpower readings of the beam using a power meter that receives the beamfrom the optical fiber of the lens system; and ending the adjusting ofthe position of the tip of the optical fiber based on the plurality ofpower readings.
 16. The storage medium of claim 15, wherein theadjusting of the position of the tip of the optical fiber includes usinga mechanical arm connecting the motor and the optical fiber.
 17. Thestorage medium of claim 15, wherein the adjusting of the position of thetip of the optical fiber includes moving the optical fiber along acenter axis of the collimating lens.
 18. The storage medium of claim 17,wherein the ending of the adjusting includes positioning the tip of theoptical fiber at a calibration position where a highest power reading ofthe plurality of power readings is detected.
 19. The storage medium ofclaim 18, further comprising securing the optical fiber in thecalibration position relative to the collimating lens.
 20. The storagemedium of claim 15, wherein the method further comprises: transmittingthe beam through an afocal telescope after transmitting through thecollimating lens; adjusting a position of one or more lenses of theafocal telescope while detecting the plurality of power readings of thebeam using the power meter; and ending the adjusting of the position ofthe one or more lenses based on the plurality of power readings.